Case-hardening steel superior in tooth surface fatigue strength, gear using the same, and method of production of the same

ABSTRACT

The present invention provides case-hardening steel superior in tooth surface fatigue strength and a gear using the same used for parts of automobiles, construction machines, industrial machines, etc., that is case-hardening steel superior in tooth surface fatigue strength containing, by wt %, C: 0.1 to 0.3%, Si: 1.0 to 2.0%, Mn: 0.3 to 2.0%, S: 0.005 to 0.05%, Cr: 1.0 to 2.6%, Mo: 0.8 to 4.0%, V: 0.1 to 0.3%, Al: 0.001 to 0.2%, and N: 0.003 to 0.03%, limiting P to 0.03% or less, and having as a balance iron and unavoidable impurities, and satisfies the following expression, 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) is 100 or more, and a gear comprised of case-hardening steel and having an X-ray diffraction half width at a depth of 50 mm from the surface of the gear of 6.4 degrees or more.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority from Japanese Patent Application Nos. 2004-377855 and 2004-377855, both filed Dec. 27, 2004 under 35 U.S.C. § 119. The entire disclosures and content of these patent applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to case-hardening steel superior in tooth surface fatigue strength and a gear using the same used for parts of automobiles, construction machines, industrial machines, etc. and a method of production of the same.

BACKGROUND ART

In automobile transmissions etc., gears comprised of mainly JIS SCr420, SCM420, and other case-hardening steels formed into gear shapes, then subjected to surface hardening by carburization quenching and tempering, etc. are used. In such gears, to increase the output of the automobiles and improve the fuel efficiency etc., lighter weight and greater gear strength have been strongly demanded. In the past, to improve the strength of the gears, technology for improving the bending fatigue strength of the tooth bases of the gears has been developed. Recently, however, along with the development of practical hard shot peening, the emphasis in increasing the strength of gears has been shifting from the bending fatigue strength of the tooth bases of gears to the tooth surface fatigue strength.

However, for improvement of the tooth surface fatigue strength, improvement of the temper softening resistance has been considered effective. In the past, as the means for improving the temper softening resistance, several technologies improving the composition of the steel materials of the gears have been proposed. For example, Japanese Unexamined Patent Publication No. 7-242994 discloses steel containing Si in an amount of 1% or less and Cr in 1.5 to 5.0%. Further, Japanese Unexamined Patent Publication No. 2001-329337 discloses steel containing Si in an amount of 0.40 to 1.50%, Mn in 0.30 to 2.00%, and Cr in 0.50 to 3.00%. Further, Japanese Patent Publication (A) No. 2003-231943 discloses steel containing Si in an amount of 0.7 to 1.5%, Cr in 0.1 to 3.0%, and Mo in 0.05 to 1.5%.

As explained above, as ingredients of steel for improving the temper softening resistance, it is known that Si, Cr, Mn, Mo, and other elements are effective, but at the present, case-hardening steel superior in tooth surface fatigue strength and gears of the same are being demanded by further improvement of the temper softening resistance.

DISCLOSURE OF THE INVENTION

In consideration of the above, an object of the present invention is to provide case-hardening steel superior in tooth surface fatigue strength and a gear using the same by more effectively improving the temper softening resistance.

As explained above, it is known that by increasing the amounts of Si, Cr, Mn, Mo, etc. in steel, it is possible to improve the temper softening resistance. The inventors discovered the following matters to further improve the temper softening resistance and thereby perfected the present invention:

(1) That in addition to the Si, Cr, Mn, and Mo, V also has an effect of improving the temper softening resistance.

(2) That the total of the five elements (Si, Cr, Mn, Mo, and V) having the effect of improving the temper softening resistance in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) is 100 or more.

Further, even if suppressing the amounts of addition of Cr, Mo, and V, solid solution hardening in the martensite structure is achieved, the required temper softening resistance is secured, and the production costs can be reduced, so 37Si(%)+18Mn(%)+10Cr(%)+31Mo(%)+21V(%) should be 100 to 150 in range.

(3) That the improvement of the temper softening resistance by the precipitation hardening by carbides of the above five elements is insufficient and that the solid solution hardening of the above five added elements in the martensite structure enables more effective improvement of the temper softening resistance.

(4) That the temper softening resistance is improved by the quenching in the carburization quenching etc. at a high temperature, causing the carbides precipitated in the material (steel) to dissolve, and making the interval from the surface of the treated gear to a depth of 50 μm have an X-ray diffraction half width of 6.4 degrees or more.

The present invention was made to achieve the above treatments and has as its gist the following:

(1) Case-hardening steel superior in tooth surface fatigue strength characterized by containing, by wt %,

-   -   C: 0.1 to 0.3%,     -   Si: 1.0 to 2.0%,     -   Mn: 0.3 to 2.0%,     -   S: 0.005 to 0.05%,     -   Cr: 1.0 to 2.6%,     -   Mo: 0.8 to 4.0%,     -   V: 0.1 to 0.3%,     -   Al: 0.001 to 0.2%, and     -   N: 0.003 to 0.03%,     -   limiting P to 0.03% or less, and     -   having a balance of iron and unavoidable impurities, and         satisfys the following expression (1).         31Si(%)+15Mn (%)+23Cr (%)+26Mo (%)+100V(%)≧100   (1)

(2) A gear superior in tooth surface fatigue strength characterized in that it comprises steel as set forth in (1) and has an X-ray diffraction half width at a depth of 50 μm from the gear surface of 6.4 degrees or more when forming the steel to a gear shape and carburizing or carbonitriding the same. The “X-ray diffraction half width” referred to here means the half width of the peak when using a micro-area X-ray residual stress measurement system (Cr lamp) to measure the α-Fe (211) plane over 60 seconds.

(3) Case-hardening steel superior in tooth surface fatigue strength as set forth in (1) wherein said steel further includes, by wt %, one or two of

-   -   Nb: 0.2% or less and     -   Ti: 0.2% or less.

(4) A gear superior in tooth surface fatigue strength as set forth in (2), wherein said gear further includes, by wt %, one or two of

-   -   Nb: 0.2% or less and     -   Ti: 0.2% or less.

(5) A gear superior in tooth surface fatigue strength as set forth in (2) or (4), characterized in that the amount of Si, Cr, Mo and V are limited to Si: 1.0-1.5%, Cr: 1.0 to 1.8%, Mo: 0.8 to 1.2%, and V: 0.10 to 0.25%, and satisfys the following expression (2) instead of the expression (1). 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%)=100˜150   (2)

(6) A method of production of a gear superior in tooth surface fatigue strength characterized by forming the steel as set forth in (5) to a gear shape, then subjecting it to vacuum carburization or vacuum carbonitridation at a heating temperature of 900 to 1050° C. in range.

BEST MODE FOR WORKING THE INVENTION

In the past, it has been known that increasing the amount of Si, Cr, Mn, Mo, and other elements in steel improves the temper softening resistance. The inventors however believed that if excessively adding these elements, large amounts of carbides would precipitate and the average size of the carbides would increase and therefore the temper softening resistance would conversely deteriorate. Therefore, the inventors thought that by dissolving Si, Cr, Mn, Mo, and other added elements in the steel, it might be possible to effectively improve the tooth surface fatigue strength of a gear.

Further, they thought that by similarly adding V as well to the steel and dissolving it in the steel, it would be possible to increase the temper softening resistance.

Therefore, the inventors postulated that by using a steel containing suitable amounts of Si, Cr, Mn, Mo, V, and other elements to make a gear and then making the added elements dissolve by high temperature carburization quenching or other quenching, it might be possible to further improve the temper softening resistance. They therefore used different steels with different amounts of addition of Si, Cr, Mn, Mo, V, and other elements to form gear shapes, then hardened the surfaces of the gears by high temperature carburization quenching and tempering so as to produce different gears and investigated the fatigue life of the tooth faces of the gears. Further, they confirmed whether the solid solution hardening by the above-mentioned added elements in the martensite structure improved the fatigue life of the tooth faces by using the X-ray diffraction half width at a depth of 50 μm from the surface of the gear as an indicator of the amount of solid solution hardening in the martensite structure and measuring the X-ray diffraction half width at a depth of 50 μm from the surface of the produced gears by a micro-area X-ray residual stress measurement system.

As a result, the following matters became clear. First, it became clear that to achieve an improvement of the tooth surface fatigue strength of a gear, just using steel increased in amounts of addition of Si, Cr, Mn, Mo, etc. is insufficient. That is, the inventors found that for improvement of the temper softening resistance, addition of V in addition to the conventional Si, Cr, Mn, or Mo is also effective, that just causing precipitation of these added elements as carbides is insufficient for improvement of the tooth surface fatigue strength of a gear, and that dissolution of the added elements into the steel effectively leads to an improvement of the tooth surface fatigue strength of a gear. From this, they guessed that metallurgically, the increase in the temper softening resistance through the precipitation hardening of the added elements is insufficient for improving the tooth surface fatigue strength of a gear and that the increase in the tempering softening resistance through solid solution hardening by the added elements in the martensite structure effectively may contribute to improvement of the tooth surface fatigue strength of a gear.

Further, they found that in steel containing C, Si, Mn, S, Cr, Mo, V, Al, N, and P in predetermined amounts and comprised of a balance of iron, unavoidable impurities, etc., a total amount of Si, Mn, Cr, Mo, and V in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) of 100 or more can more effectively improve the temper softening resistance

Further, even if suppressing the amounts of addition of Cr, Mo, and V and making 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+21V (%) 100 to 150 in range, solid solution hardening of the martensite structure is achieved, the required temper softening resistance is secured, and production costs can be reduced.

Further, it became clear that when using this steel as a material, forming it to a gear shape, then subjecting the surface of the gear to vacuum carburization, carbonitridation, and other surface hardening, a gear having an X-ray diffraction half width at a depth of 50 μm from the surface of the gear of 6.4 degrees or more is further improved in temper softening resistance, that is, has a superior tooth surface fatigue strength.

From the above, steel containing C, Si, Mn, S, Cr, Mo, V, Al, N, and P in predetermined amounts and comprised of a balance of iron, unavoidable impurities, etc., having a total of Si, Mn, Cr, Mo, and V in 31Si(%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) of 100 or more, and further having a total of Si, Mn, Cr, Mo and V in 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+21V (%) of 100 to 150 can be said to be useful as case-hardening steel superior in tooth surface fatigue strength.

Further, by selecting gears having an X-ray diffraction half width at a depth of 50 μm from the surface of the gear of 6.4 degrees or more from gears obtained by using the above-mentioned case-hardening steel as a material for shaping gears, then subjecting the surfaces of the gears to vacuum carburization, carbonitridation, and other surface hardening, it is suggested that gears superior in tooth surface fatigue strength can be obtained. Therefore, it is believed that the thus obtained gears rise in temperature near the surface of the tooth faces to about 300° C. due to the heat of friction generated due to contact of the drive faces and driven faces of the gears at a high facial pressure accompanied with sliding and have resistance even with the temper softening arising as a result and that further this can greatly contribute to higher output, improved fuel efficiency, etc. in automobiles, construction machines, industrial machines, etc. Note that while the gears superior in tooth surface fatigue strength according to the present invention can be obtained in the above-mentioned way, they may also be obtained by carburization or carbonitridation, then shot peening, subzero cooling, WPC, WJP, etc. Due to this, it becomes possible to make the residual austenite at the surface of the gear transform to martensite and increase the temper softening resistance.

Next, the ranges of the wt % of the chemical ingredients included in the steel of the present invention (case-hardening steel) will be explained.

-   -   C: 0.1 to 0.3%

C is an element essential for maintaining the strength of the steel. Its content determines the strength of the core part and also affects the effective hardened layer depth. Therefore, in the present invention, the lower limit of the amount of C was made 0.1% to secure the core strength. However, if the content is too great, the toughness falls, so 0.3% was made the upper limit.

-   -   Si: 1.0 to 2.0%

Si is an element effective for improving the temper softening resistance. Addition of 1.0% gives this effect. Therefore, in the present invention, the lower limit of the amount of Si was made 1.0%. However, if the content is over 2.0%, the carburization ability deteriorates, so 2.0% was made the upper limit.

-   -   P Mn: 0.3 to 2.0%

Mn is an element effective for improving the hardenability and further is an element effective for improving the temper softening resistance. Further, it also has the action of immobilizing the impurity element S unavoidably contained in the steel as MnS and thereby rendering it harmless. Therefore, as the amount of Mn, 0.3% or more is believed necessary. Therefore, in the present invention, the lower limit of the amount of Mn was made 0.3%. However, if the content is over 2.0%, this ends up increasing and stabilizing the residual austenite in the carburized layer to an extent unable to be prevented even if performing subzero cooling and the temper softening resistance conversely deteriorates, so 2.0% was made the upper limit.

-   -   S: 0.005 to 0.05%

S is an impurity element unavoidably included, but from the viewpoint of the machinability must be included in an amount of 0.005% or more. Therefore, in the present invention, the lower limit of the amount of S was made 0.005%. However, if the content is over 0.05%, the forgeability is inhibited, so 0.05% was made the upper limit.

-   -   Cr: 1.0 to 2.6%

Cr is an element effective for improving the hardenability and is an element effective for improving the temper softening resistance. Addition in 1.0% or more gives this effect. Therefore, in the present invention, the lower limit of the amount of Cr was made 1.0%. However, if the content exceeds 2.6%, the Cr carbides present in the material will not completely dissolve even with high temperature carburization and the temper softening resistance will conversely deteriorate, so 2.6% was made the upper limit. Note that to completely prevent the occurrence of coarse grains in the carburization, Cr is preferably 1.0 to 1.8%.

-   -   Mo: 0.8 to 4.0%

Mo is an element effective for improving the hardenability and is an element effective for improving the temper softening resistance. Addition in 0.8% or more gives this effect. Therefore, in the present invention, the lower limit of the amount of Mn was made 0.8%. However, if the content is over 4.0%, the Mo carbides present in the material cannot completely dissolve even with high temperature carburization and the temper softening resistance conversely deteriorates, so 4.0% was made the upper limit. Note that to completely prevent the occurrence of coarse grains in the carburization, Mo is preferably 1.0 to 1.2%.

-   -   V: 0.1 to 0.3%

V is an element effective for improving the temper softening resistance. Addition of 0.1% or more gives this effect. Therefore, in the present invention, the lower limit of the amount of V was made 0.1%. However, if the content is over 0.3%, the V carbides present in the material cannot completely dissolve in solid solution even by high temperature carburization and the temper softening resistance conversely deteriorates, so 0.3% was made the upper limit. Note that to completely prevent the occurrence of coarse grains in the carburization, V is preferably 0.1 to 0.25%.

-   -   Al: 0.001 to 0.2%

Al has the effect of refinement of the crystal grains due to the formation of compounds with N, so 0.001% or more is considered necessary. Therefore, in the present invention, the lower limit of the amount of Al was made 0.001%. However, if over 0.2%, the machineability is remarkably inhibited, so 0.2% was made the upper limit.

-   -   N: 0.003 to 0.03%

N is an unavoidably included element, but also has the effect of refinement of the crystal grains by formation of compounds with Al and N, so 0.003% or more is considered necessary. Therefore, in the present invention, the lower limit of the amount of N was made 0.003%. However, if the content is over 0.03%, the forgeability is remarkably inhibited, so 0.03% was made the upper limit.

-   -   P: limited to 0.03% or less

P is an unavoidably included impurity element. It precipitates at the grain boundaries and lowers the toughness, so has to be limited to 0.03% or less. Therefore, in the present invention, the amount of P was limited to 0.03% or less.

In addition, for the purpose of further refinement of the crystal grains or preventing coarsening of the crystal grains in the steel of the present invention, it is also possible to further add Nb, Ti, etc. in addition to the above-mentioned chemical ingredients. In this case, these elements are preferably included in the following ranges not inhibiting the productivity of the hot rolling, hot forging, cutting, etc.

-   -   Nb: 0.2% or less and Ti: 0.2% or less, one or both

Nb and Ti have the effect of refinement of the crystal grains due to the formation of compounds with N, so inclusion of one or both of Nb and Ti is preferable. However, even if each element is included in an amount of over 0.2%, the effect of refinement of the crystal grains becomes saturated and the economicalness is impaired, so 0.2% was made the upper limit.

Next, the total amount of the Si, Mn, Cr, Mo, and V in the steel of the present invention will be explained. In the present invention, the total amount of Si, Mn, Cr, Mo, and V in the following formula being 100 or more is an essential condition.

This, as explained above, is based on intensive research and development by the inventors and as a result the discovery that when the total amount of the Si, Mn, Cr, Mo, and V in the following expression is 100 or more, a gear superior in tooth surface fatigue strength can be obtained. Note that at the left side in the expression, the coefficients of the elements of Si, Mn, Cr, Mo, and V differ because the extents by which the elements contribute to improvement of the temper softening resistance differ. 31Si(%)+15Mn(%)+23Cr(%)+26Mo(%)+100V(%)≧100

Further, in the present invention, a total amount of Si, Mn, Cr, Mo, and V in the following expression of 100 to 150 in range is an essential condition. This is because, as explained above, the inventors engaged in intensive research and as result found that when the total amount of the Si, Mn, Cr, Mo, and V in the above expression is 100 or more, a gear superior in tooth surface fatigue strength can be obtained. If using a steel material of a high alloy composition of over 150 in the above expression, the starting point of the martensite transformation falls. Due to this, the amount of residual austenite after vacuum carburization exceeds 20%. Compared with martensite, residual austenite is softer. Due to this, a remarkable drop in the strength of the surface of the gear is caused. Therefore, in the present invention, a total amount of Si, Mn, Cr, Mo, and V in the following expression of 150 or less was made a condition. Note that at the left side in the following expression, the coefficients of the elements of Si, Mn, Cr, Mo, and V differ because the extents by which the elements contribute to the improvement of the temper softening resistance differ. 37Si (%)+18Mn (%)+10Cr(%)+31Mo (%)+201V (%)=100 to 150

Next, the reason for the gear according to the present invention having an X-ray diffraction half width at a depth of 50 μm from the surface of the gear of 6.4 degrees or more being made a condition will be explained.

By satisfying the above formula and securing an X-ray diffraction half width at a depth of 50 μm from the surface of the gear of 6.4 degrees or more, a gear superior in tooth surface fatigue strength was realized. Even if using steel satisfying just the above expression to form a gear shape and subjecting this to carburization quenching and tempering at a general 930° C., the X-ray diffraction half width at a depth of 50 μm from the surface of the gear will not necessarily become 6.4 degrees or more. The inventors believed that therefore selection of a surface hardening giving an X-ray diffraction half width of 6.4 degrees or more was crucial. Further, at a stage before the surface hardening, some of the Mn, Cr, Mo, and V remains as carbides, but as the contents of Mo, V, etc. become greater, with carburization at the general 930° C., the dissolution of the carbides becomes insufficient and an X-ray diffraction half width of 6.4 degrees or more becomes impossible to secure. Therefore, it is believed necessary to cause the carbides to dissolve at a carburization temperature of preferably 950° C. or more, in some cases 1000° C. or more. Further, as the value at the left side in the above expression becomes greater, the amount of residual austenite tends to gradually become greater. Along with this, the X-ray diffraction half width tends to become smaller. For this reason, when the value of the above expression is 130 or more, it is considered effect to further perform subzero cooling or shot peening to transform the residual austenite to martensite and make the X-ray half width 6.4 degrees or more.

Therefore, in the present invention, an X-ray diffraction half width at a depth of 50 μm from the gear surface of 6.4 degrees or more was made a condition. Note that the above-mentioned X-ray diffraction half width means the half width of the peak when using a micro-area X-ray residual stress measurement system (Cr lamp) to measure the α-Fe (211) plane over 60 seconds.

Next, the reason for vacuum carburizing the tooth surfaces of the gear at a heating temperature in a range of 900 to 1050° C. after using above-mentioned steel material to form a gear shape will be explained.

A carburization temperature of less than 900° C. is insufficient for making the carbides dissolve in the material containing 100 or more of the elements in the above formula (steel). 900° C. or more, preferably 950° C. or more, is necessary. Therefore, in the present invention, the lower limit of the carburization temperature was made 900° C. However, if the carburization temperature exceeds 1050° C., the problem of coarse grains arises, so 1050° C. was made the upper limit.

However, in general, as the method of carburization, gas carburization and vacuum carburization are broadly used. The inventors investigated this and found that with gas carburization, the fine amount of oxygen contained in the carrier gas causes grain boundary oxidation of about 10 μm at the surface of the gear resulting in a drop in the strength, so vacuum carburization must be applied. Therefore, in the present invention, treating the tooth surfaces of the gear shape by vacuum carburization was made a condition.

Further, in the present embodiment, the inventors used the above-mentioned steel material as a material to form a gear shape, then subjected this to vacuum carburization at a heating temperature of 900 to 1050° C. in range so as to produce a gear superior in tooth surface fatigue strength, but even if performing broadly used treatment after the above vacuum carburization, for example, shot peening, WPC, WJP, subzero cooling, etc., the effect of the present invention will not be inhibited, so these treatments may be performed after the vacuum carburization.

EXAMPLES Example 1

Below, the present invention will be explained in more detail by examples. Note that these examples are for explaining the present invention and do not limit the scope of the present invention.

Hot rolled steel materials having the chemical compositions shown in Table 1 were spheroidally annealed to secure machineability, then were used to fabricate drive gears and driven gears with pitch circle diameters of 65.8 mm, modules of 1.5, and 35 teeth (Test Nos. 1 to 15). TABLE 1 31 Si + Test Chemical composition (wt %) 15 Mn + 23 Cr + No. C Si Mn P S Cr Mo V Al N Others 26 Mo + 100 V 1 Inv. ex. 0.21 1.30 0.35 0.008 0.012 1.53 1.01 0.20 0.038 0.015 127 2 Inv. ex. 0.21 1.31 0.36 0.028 0.049 1.53 1.02 0.11 0.035 0.014 119 3 Inv. ex. 0.10 1.40 0.35 0.006 0.013 2.53 1.01 0.10 0.001 0.010 Ti:0.028 143 4 Inv. ex. 0.29 1.41 0.34 0.007 0.014 2.50 1.01 0.15 0.036 0.018 148 5 Inv. ex. 0.20 1.98 0.36 0.007 0.013 2.01 1.05 0.14 0.002 0.009 Ti:0.025 154 6 Inv. ex. 0.20 1.40 0.31 0.006 0.005 2.54 1.01 0.11 0.035 0.016 144 7 Inv. ex. 0.20 1.41 1.98 0.008 0.014 2.53 1.02 0.29 0.197 0.016 187 8 Inv. ex. 0.21 1.30 0.35 0.008 0.012 2.60 1.00 0.20 0.038 0.262 151 9 Inv. ex. 0.20 1.05 0.35 0.007 0.012 2.43 3.95 0.16 0.036 0.015 212 10 Inv. ex. 0.21 1.03 0.36 0.009 0.012 2.52 0.99 0.50 0.033 0.016 Nb:0.031 171 11 Comp. 0.21 2.54 0.36 0.007 0.014 1.51 1.01 0.15 0.035 0.013 160 ex. 12 Comp. 0.20 2.02 0.36 0.007 0.013 2.01 1.01 0.16 0.035 0.013 157 ex. 13 Comp. 0.21 0.25 0.78 0.014 0.018 1.16 1.02 0.10 0.035 0.013 83 ex. 14 Comp. 0.20 0.25 0.78 0.015 0.015 1.23 1.02 0.22 0.035 0.013 96 ex. 15 Comp. 0.21 1.03 0.36 0.009 0.012 2.52 1.51 0.10 0.033 0.016 145 ex.

Next, the surface hardening explained below was performed under working conditions giving an effective hardened layer depth of the gear of 0.6 mm. In Test Nos. 1 to 3, 5, 6, 11 to 15, vacuum carburization quenching was performed at 1000° C., then tempering was performed at 200° C. for 90 minutes. In Test No. 7, vacuum carburization quenching was performed at 1000C, subzero cooling was performed by liquid nitrogen for 60 minutes, then finally tempering was performed at 200° C. for 90 minutes. In Test No. 4, gas carburization at 950° C. for 120 minutes and carbonitridation at 860° C. for 30 minutes were successively performed, then quenching was performed, then tempering was performed at 200° C. for 90 minutes, then shot peening was performed at an arc height of 1.0. In Test Nos. 8 and 9, vacuum carburization quenching was performed at 1050° C., subzero cooling was performed by liquid nitrogen for 60 minutes, and finally tempering was performed at 200° C. for 90 minutes. In Test No. 10, vacuum carburization was performed at 1050° C., then tempering was performed at 200° C. for 90 minutes, and finally shot peening was performed at an arc height of 1.0.

Then, the inventors evaluated the amounts of increase of the temper softening resistance due to the solution hardening by the Si, Cr, Mn, Mo, and other added elements for the above-mentioned treated Test Nos. 1 to 15. Note that temper softening resistance is usually evaluated by using a microvicker's hardness meter etc. to measure the hardness in a micro area, but with this method of evaluation, the amount of hardening due to precipitation and the amount of hardening due to solid solution cannot be differentiated, so it is not possible to measure only the amount of hardening due to solid solution. Therefore, in this embodiment, based on the discovery that the amount of increase due to the solid solution hardening in the martensite structure is important for improving the tooth surface fatigue strength of a gear, the inventors measured, the X-ray diffraction half width at a depth of 50 μm from the gear surface of the gear as an indicator of the amount of increase due to the solid solution hardening in the martensite structure by a micro-area X-ray residual stress measurement system so as to evaluate the amount of increase of the temper softening resistance. Note that the X-ray diffraction half width at a depth of 50 μm from the gear surface of Test Nos. 1 to 15 was found by using a micro-area X-ray residual stress measurement system (Cr lamp) to measure the half width of the peak for the α-Fe (211) plane over 60 seconds.

Further, the inventors investigated the fatigue life of the tooth surfaces of Test Nos. 1 to 15 by using a power circulating type gear fatigue tester to investigate the lifetime (X) at a test load of 200N·m. Note that the lifetime was measured by detecting the vibration accompanying chipping of the tooth face. The above test results are shown in Table 2. TABLE 2 X-ray diffraction Test Surface half width Test results No. hardening (degree) Lifetime (X) 1 Inv. ex. [1] 6.44 1,394,645 2 Inv. ex. [1] 6.48 1,275,430 3 Inv. ex. [1] 6.82 1,421,972 4 Inv. ex. [2], [3] 6.63 1,381,593 5 Inv. ex. [1] 6.51 1,291,377 6 Inv. ex. [1] 6.79 1,124,314 7 Inv. ex. [1], [4] 7.13 1,571,850 8 Inv. ex. [1], [4] 7.57 1,948,836 9 Inv. ex. [1], [4] 7.63 2,451,598 10 Inv. ex. [1], [3] 6.55 2,022,445 11 Comp. ex. [1] 3.50 11,582 12 Comp. ex. [1] 5.09 700,228 13 Comp. ex. [1] 7.02 527,288 14 Comp. ex. [1] 6.61 922,487 15 Comp. ex. [1] 5.27 4,993 [1] Vacuum carburization quenching and tempering [2] Carbonitridation quenching and tempering [3] Shot peening [4] Subzero cooling

From these results, in the Invention Test Nos. 1 to 10, it was learned that the gears had lifetimes of 1,000,000 or more, so had superior tooth surface fatigue strengths. This was believed due to the facts that the wt % of the chemical ingredients included in the steel were in the predetermined ranges (C of 0.1 to 0.3% in range, Si of 1.0 to 2.0% in range, Mn of 0.3 to 2.0% in range, S of 0.005 to 0.05% in range, Cr of 1.0 to 2.6% in range, Mo of 0.8 to 4.0% in range, V of 0.1 to 0.3% in range, Al of 0.001 to 0.2% in range, N of 0.003 to 0.03% in range, and P of 0.03% or less), the totals of the Si, Mn, Cr, Mo, and V in the steel in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V(%) were 100 or more, and the X-ray diffraction half widths at a depth of 50 μm from the gear surface of the gear were 6.4 or more.

As opposed to this, in Comparative Example Test Nos. 11, 12, the gears had total amounts of Si, Mn, Cr, Mo, and V in the steel in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) of 100 or more, but had insufficient lifetimes of less than 1,000,000. This was believed due to the high Si contents causing poor carburization which in turned caused the concentration of C at the gear surfaces to drop to 0.3 to 0.4% and therefore cause the X-ray diffraction half widths to become less than 6.4 degrees.

In Comparative Example Test Nos. 13 and No. 14, the gears had X-ray diffraction half widths of 6.4 or more, but had insufficient lifetimes of less than 1,000,000. This was believed to be probably due to the total amounts of the Si, Mn, Cr, Mo, and V in the steel in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) being less than 100 and therefore causing the temper softening resistances to drop.

In Comparative Example Test No. 15, the gear had a total of the Si, Mn, Cr, Mo, and V in the steel in 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%) of 100 or more, but had an insufficient lifetime of less than 1,000,000. This was examined after the test. As a result, it was confirmed that in Comparative Example Test No. 15, a large amount of residual austenite remained. Therefore, in Test No. 15, it was believed that the large amount of residual austenite present resulted in the X-ray diffraction half width becoming less than 6.4 degrees and caused a drop in the temper softening resistance. Therefore, in this comparative example, it was believed that by further performing subzero cooling, shot peening, or other treatment, it would be possible to make the residual austenite transform to martensite, treat the residual austenite, and make the X-ray half width 6.4 degrees or more.

Example 2

Hot rolled steel materials having the chemical compositions shown in Table 3 were spheroidally annealed to secure machineability, then were used to fabricate drive gears and driven gears with pitch circle diameters of 65.8 mm, modules of 1.5, and 35 teeth (Test Nos. 1 to 17). TABLE 3 37 Si + Test Chemical composition (wt %) 18 Mn + 10 Cr + No. C Si Mn P S Cr Mo V Al N Others 31 Mo + 201 V 16 Inv. ex. 0.21 1.30 0.35 0.008 0.012 1.53 1.01 0.20 0.038 0.015 141 17 Inv. ex. 0.20 1.02 0.35 0.020 0.019 1.32 0.80 0.10 0.035 0.012 102 18 Inv. ex. 0.15 1.40 0.35 0.006 0.013 1.54 1.01 0.10 0.001 0.011 Ti:0.029 125 19 Inv. ex. 0.25 1.39 0.34 0.007 0.014 1.50 1.01 0.15 0.038 0.018 134 20 Inv. ex. 0.20 1.49 0.36 0.007 0.013 1.10 0.80 0.10 0.001 0.009 Ti:0.023 118 21 Inv. ex. 0.20 1.40 0.31 0.006 0.005 1.02 0.81 0.10 0.033 0.015 113 22 Inv. ex. 0.19 1.01 1.95 0.008 0.014 1.01 0.85 0.20 0.039 0.015 149 23 Inv. ex. 0.21 1.30 0.35 0.008 0.012 1.74 0.85 0.21 0.035 0.019 149 24 Inv. ex. 0.20 1.04 0.36 0.009 0.015 1.51 1.19 0.15 0.034 0.014 127 25 Inv. ex. 0.20 1.12 0.36 0.008 0.011 1.49 0.99 0.25 0.033 0.016 Nb:0.031 144 26 Comp. 0.19 1.55 0.35 0.008 0.016 1.50 1.00 0.15 0.033 0.013 140 ex. 27 Comp. 0.21 0.25 0.78 0.014 0.018 1.16 1.02 0.10 0.035 0.013 87 ex. 28 Comp. 0.20 0.25 0.78 0.015 0.015 1.23 0.95 0.17 0.035 0.013 99 ex. 29 Comp. 0.19 1.01 1.90 0.009 0.015 1.55 1.20 0.20 0.033 0.016 164 ex. 30 Comp. 0.21 1.30 0.35 0.008 0.012 1.53 1.01 0.20 0.038 0.015 141 ex. 31 Comp. 0.21 1.30 0.35 0.008 0.012 1.53 1.01 0.20 0.038 0.015 141 ex. 32 Comp. 0.21 1.30 0.35 0.008 0.012 1.53 1.01 0.20 0.038 0.015 141 ex.

Next, the surface hardening treatment explained below was performed under working conditions giving an effective hardened layer depth of the gear of 0.6 mm. In Test Nos. 16, 18, 20 to 22, 24, 26, and 29, vacuum carburization quenching was performed at 1000° C., then tempering was performed at 200° C. over 90 minutes. In Test No. 17, vacuum carburization quenching was performed at 900° C., then tempering was performed at 200° C. over 90 minutes. In Test Nos. 19, 23, 27, and 28, vacuum carburization quenching was performed at 950° C., then tempering was performed at 200° C. over 90 minutes. In Test No. 25, vacuum carburization quenching was performed at 1050° C., then tempering was performed at 200° C. over 90 minutes. In Test No. 30, vacuum carburization quenching was performed at 950° C., then tempering was performed at 200° C. over 90 minutes. In Test No. 31, high carbon carburization treatment by gas carburization quenching at 950° C. and a carbon potential of 1.3 followed by gas carburization quenching by a carbon potential of 0.95 was performed, then tempering was performed at 200° C. over 90 minutes. In Test No. 32, vacuum carburization quenching was performed at 890° C., then tempering was performed at 200° C. over 90 minutes.

After the tempering, the inventors investigated the fatigue life of the tooth faces in Test Nos. 16 to 32 by using a power circulating type gear fatigue tester to investigate the lifetime (X) at a test load of 200N·m. Note that the lifetime was measured by detecting the vibration accompanying chipping of the tooth face.

Further, the inventors evaluated the amount of increase of the temper softening resistance due to the solid solution hardening of Si, Cr, Mn, Mo, and other added elements for Test Nos. 16 to 32. Note that the temper softening resistance was evaluated normally by using a microvicker's hardness meter etc. to measure the hardness in a micro area, but with this method of evaluation, the increase in hardness due to precipitation hardening also ends up being included and therefore only the increase in hardness due to solid solution hardening cannot be measured. Therefore, in this example, based on the discovery that the increase in hardness due to solution hardening in the martensite structure is important in improving the tooth surface fatigue strength of a gear, the inventors used an optical microscope, scan type electron microscope, etc. to examine the microstructure and investigate if there were coarse carbides in the interval at a depth of 50 μm from the surface of the gears produced as an indicator of the amount of increase of hardness due to the solid solution hardening in the martensite structure, more specifically, if the average size of the carbides was less than 1 μm. Note that the average size of the carbides was measured as explained next. First, after the test, the gear was cut and buried in a resin to prepare a sample which was then mirror polished. Then, the polished surface of the sample was etched by a Nytal corrosive solution, a scan type electron microscope was used to randomly observe carbides up to a depth of 50 μm from the gear surface, and the values of the sizes of the carbides observed were arithmetically averaged.

Further, it is known that if the starting point of the martensite transformation falls at the quenching stage, the amount of residual austenite increases and a drop in the strength is caused. Therefore, the ratio of the amount of the residual austenite at a depth of 50 μm from the surface of the gear in each of Test Nos. 16 to 32 was found by observation of the structure by a microscope. The above test results are shown in Table 4. TABLE 4 Test results Amount of residual Average size of austenite from carbides in interval tooth surface Test Carburization from gear surface to to 50 μm No. temperature 50 μm depth depth Lifetime (X) 16 Inv. ex. 1000° C. Average size less 10% 1,394,645 than 1 μm 17 Inv. ex.  900° C. Average size less 10% 1,284,625 than 1 μm 18 Inv. ex. 1000° C. Average size less 15% 1,226,956 than 1 μm 19 Inv. ex.  950° C. Average size less 16% 1,364,485 than 1 μm 20 Inv. ex. 1000° C. Average size less 13% 1,248,652 than 1 μm 21 Inv. ex. 1000° C. Average size less 13% 1,052,363 than 1 μm 22 Inv. ex. 1000° C. Average size less 15% 1,327,421 than 1 μm 23 Inv. ex.  950° C. Average size less 20% 1,462,147 than 1 μm 24 Inv. ex. 1000° C. Average size less 18% 1,322,574 than 1 μm 25 Inv. ex. 1050° C. Average size less 14% 1,311,667 than 1 μm 26 Comp. 1000° C. Average size less  0% 10,285 ex. than 1 μm 27 Comp.  950° C. Average size less 15% 527,288 ex. than 1 μm 28 Comp.  950° C. Average size less 13% 922,487 ex. than 1 μm 29 Comp. 1000° C. Average size less 30% 965,477 ex. than 1 μm 30 Comp. Gas carburization Average size less 18% 501,448 ex. than 1 μm 31 Comp. High carbon Average size more  7% 653,211 ex. carburization than 15 μm 32 Comp.  890° C. Average size more 15% 844,856 ex. than 3 μm

From these results, it became clear that since Test Nos. 16 to 25 of the examples of the present invention have lifetimes of 1,000,000 or more, they have superior tooth surface fatigue strength. This is believed to be due to the facts that the wt % of the chemical ingredients included in the steel material are in the predetermined ranges (C of 0.15 to 0.25% in range, Si of 1.0 to 1.5% in range, Mn of 0.3 to 2.0% in range, S of 0.005 to 0.02% in range, Cr of 1.0 to 1.8% in range, Mo of 0.8 to 1.2% in range, V of 0.10 to 0.25% in range, Al of 0.001 to 0.04% in range, N of 0.003 to 0.02% in range, and P of 0.02% or less), the total amount of the Si, Mn, Cr, Mo, and V in the steel material in 37Si (%)+18Mn (%)+10Cr (%)+3 1Mo (%)+201V (%) is 100 to 150 in range, vacuum carburization is performed in a temperature range of 900 to 1050° C., and other conditions are satisfied and thereby the amount of precipitation of carbides at the surface of the gear is reduced and the amount of residual austenite can be suppressed to within 20%.

As opposed to this, in Comparative Example Test No. 26, despite the small amount of precipitation of carbides and the residual austenite being 20% or less, the gear had an insufficient lifetime of less than 1,000,000. The inventors investigated this after the test and as a result learned that the poor carburization caused the concentration of C at the gear surface to become a low 0.3%. From this, it was believed that if the Si content of the steel material is over 1.5%, the carburization ability deteriorates.

In Comparative Example Test No. 27 and No. 28 as well, the gears had small amounts of precipitation of carbides and amounts of residual austenite of 20% or less, yet had insufficient lifetimes of less than 1,000,000. This was believed to be possibly due to the fact that the total amounts of the Si, Mn, Cr, Mo, and V in the steel materials in 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%) were less than 100. In Comparative Example Test No. 29, the gear had an insufficient lifetime of less than 1,000,000 and did not have tooth surface fatigue strength. This was believed because the total amount of the Si, Mn, Cr, Mo, and V in the steel material in 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%) exceeded 150 and the amount of residual austenite was a large 30% and therefore the strength fell.

In Comparative Example Test No. 30, the gear had a small amount of precipitation of carbides and an amount of residual austenite of 20% or less, yet had an insufficient lifetime of less than 1,000,000. It was learned that this was due to granular boundary oxidation of about 10 μm at the gear surface of the gear and that this formed starting points of fracture. From this, it was believed that with gas carburization quenching at 950° C., the fine amount of oxygen contained in the carrier gas causes grain boundary oxidation at the tooth surfaces of the gear and invites a drop in strength, so superior tooth surface fatigue strength cannot be obtained.

In Comparative Example Test No. 31, the gear had an insufficient lifetime of less than 1,000,000 and did not have superior tooth surface fatigue strength. The inventors investigated this after the test. As a result, they found that a troostite structure was observed and the quenching was insufficient. This insufficient quenching was believed due to the Cr, Mo, and V dissolving in solid solution in the carbides of an average size of 15 μm or so formed by the high carbon carburization and thereby those elements becoming insufficient in the steel material matrix. Due to this, it was believed that with high carbon carburization by gas carburization quenching at 950° C., a superior tooth surface fatigue strength cannot be obtained.

In Comparative Example Test No. 32 subjected to vacuum carburization quenching at 890° C., it became clear that the gear had an insufficient lifetime of less than 1,000,000 and that a large number of carbides with an average size of 3 μm or more remained at the interval from the surface of the gear to a depth of 50 μm. From this, it was believed that with vacuum carburization quenching at 890° C., a large number of carbides with an average size of 3 μm or more are formed and due to this superior tooth surface fatigue strength cannot be obtained.

As explained above, it is possible to more effectively improve the temper softening resistance and thereby provide case-hardening steel superior in tooth surface fatigue strength and a gear using the same and possible to use these to greatly contribute to the higher output and improved fuel efficiency of automobiles, construction machines, industrial machines, etc. 

1-8. (canceled)
 9. Case-hardening steel superior in tooth surface fatigue strength characterized by containing, by wt %, C: 0.1 to 0.3%, Si: 1.0 to 2.0%, Mn: 0.3 to 2.0%, S: 0.005 to 0.05%, Cr: 1.0 to 2.6%, Mo: 0.8 to 4.0%, V: 0.1 to 0.3%, Al: 0.001 to 0.2%, and N: 0.003 to 0.03%, limiting P to 0.03% or less, and having a balance of iron and unavoidable impurities, wherein satisfying the following expression (1). 31Si (%)+15Mn (%)+23Cr (%)+26Mo (%)+100V (%)≧100   (1)
 10. Case-hardening steel superior in tooth surface fatigue strength as set forth in claim 9 wherein said steel further includes, by wt %, one or two of Nb: 0.2% or less and Ti: 0.2% or less
 11. A gear superior in tooth surface fatigue strength characterized in that it comprises steel as set forth in claim 9 and has an X-ray diffraction half width at a depth of 50 μm from the gear surface of 6.4 degrees or more when forming the steel to a gear shape and carburizing or carbonitriding the same, the “X-ray diffraction half width” referred to here meaning the half width of the peak when using a micro-area X-ray residual stress measurement system (Cr lamp) to measure the α-Fe (211) plane over 60 seconds.
 12. A gear superior in tooth surface fatigue strength as set forth in claim 11, wherein said gear further includes, by wt %, one or two of Nb: 0.2% or less and Ti: 0.2% or less
 13. A gear superior in tooth surface fatigue strength as set forth in claim 11, characterized in that the amount of Si, Cr, Mo and V are limited to Si: 1.0-1.5%, Cr: 1.0 to 1.8%, Mo: 0.8 to 1.2%, and V: 0.10 to 0.25%, and satisfies the following expression (2) instead of the expression (1): 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%)=100˜150   (2)
 14. A gear superior in tooth surface fatigue strength as set forth in claim 12, characterized in that the amount of Si, Cr, Mo and V are limited to Si: 1.0-1.5%, Cr: 1.0 to 1.8%, Mo: 0.8 to 1.2%, and V: 0.10 to 0.25%, and satisfies the following expression (2) instead of the expression (1): 37Si (%)+18Mn (%)+10Cr (%)+31Mo (%)+201V (%)=100˜150   (2)
 15. A method of production of a gear superior in tooth surface fatigue strength characterized by forming the steel as set forth in claim 14 to a gear shape, then subjecting it to vacuum carburization or vacuum carbonitridation at a heating temperature of 900 to 1050° C. in range. 